This is 'Niecza',
which is a Perl 6 implementation focusing on optimization and efficient implementation research.
It targets the Common Language Runtime (ECMA-335; implementations are "Mono" and ".NET").

To run on Mono: 2.6.4 and later are well-tested,
though success has been reported with 2.4.x as well.
ggoebel on #perl6 reports that 2.6.7 fails with a bus error on OS X 10.4 x86,
but 2.6.4 works; 2.6.7 is fine on other platforms.

2.8.2 or later is recommended for best performance; you should use RUN_CLR=mono-sgen in this case.
2.10 (latest stable) is also tested.

Debian and derivatives have a modular mono packaging; you should install 'mono-complete'.
(The other packages exist for the sake of Debian-packaged programs.)

To run on Windows/Mono (2.8.2 and 2.10) and other systems,
download the mono installers from http://go-mono.org/

To run on Windows/Microsoft .NET 3.5 (2.0 runtime with 3.5 extensions) or .NET 4.0,
install the .NET runtime from http://goo.gl/PMKvB

NOTE: Niecza does NOT currently support Windows line endings.
Source code (including the setting) must be encoded in Unix format,
that is,
linefeed only.

Notes: The first time the compiler is run, the setting must be compiled, which adds about a minute on most hardware to the runtime. The second run will be much faster. Also, the recompilation checker is not robust to file format changes; it may be necessary to clean temporary files after updates.

Building on Windows currently requires the Cygwin environment with Mono's gmcs.exe in your Cygwin shell's $PATH, though in principle .NET's csc.exe could be made to work eventually, if it doesn't already.